feat: allow switching embedded signup whatsapp embedded inboxes to manual setup (#14975)

Since embedded signup has been disabled on production, WhatsApp inboxes
that were created through it have no way to be managed going forward.
This PR lets admins transfer an embedded signup inbox to a manual Cloud
API setup: the inbox settings Configuration tab now shows the webhook
verification token and a "Switch to Manual Setup" form (pre-populated
with the Phone Number ID, Business Account ID, and API key stored during
the embedded signup journey) instead of the old Reconfigure button.

Saving the form updates the channel's `provider_config` without the
`source: embedded_signup` marker, so the inbox becomes a regular
manually-configured WhatsApp Cloud inbox. The backend re-validates the
submitted credentials against Meta before accepting the change.

## How to test

1. Open the settings page of a WhatsApp inbox created via embedded
signup → Configuration tab.
2. The Reconfigure button is gone; you see the webhook verification
token and a Switch to Manual Setup form pre-filled with the stored
credentials.
3. Configure the webhook in your own Meta app using the verification
token, enter a permanent access token from that app, and click "Switch
to Manual Setup".
4. On success the page switches to the standard manual configuration
view (verify token, API key update), and messaging continues to work
with the new credentials. Invalid credentials are rejected with an
error.

## What changed

- `ConfigurationPage.vue`: replaced the embedded-signup Reconfigure
section (and the hidden reauthorize component) with the manual transfer
form.
- New `WHATSAPP_MANUAL_TRANSFER_*` translation keys.

---------

Co-authored-by: Muhsin <12408980+muhsin-k@users.noreply.github.com>
Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>
Co-authored-by: iamsivin <iamsivin@gmail.com>

app/models/channel/whatsapp.rb

@@ -33,6 +33,7 @@ class Channel::Whatsapp < ApplicationRecord
   validate :validate_provider_config
 
   after_create :sync_templates
+  after_update_commit :log_credentials_transfer, if: :saved_change_to_provider_config?
   before_destroy :teardown_webhooks
   after_commit :setup_webhooks, on: :create, if: :should_auto_setup_webhooks?
 
@@ -129,6 +130,15 @@ class Channel::Whatsapp < ApplicationRecord
     errors.add(:provider_config, 'Invalid Credentials') unless provider_service.validate_provider_config?
   end
 
+  # Logs only credential changes, so config-only saves (e.g. calling toggles) stay silent.
+  def log_credentials_transfer
+    before, after = saved_change_to_provider_config
+    keys = %w[api_key phone_number_id business_account_id]
+    return if before.nil? || before.values_at(*keys) == after.values_at(*keys)
+
+    Rails.logger.info("[WHATSAPP_MANUAL_TRANSFER] success account_id=#{account_id} channel_id=#{id}")
+  end
+
   def perform_webhook_setup
     webhook_setup_service.perform
   end